Capacitor/Transistor Mystery for an electronics novice.

Thread Starter

TarenF

Joined Jul 10, 2018
6
Hello everyone, I'm brand new here and hope I've chosen an appropriate section to post this in.

I'm learning about circuits and curiosity about transistors made me wonder if I would get a delayed response by hooking 5V to the transistors trigger pin through a 100uF capacitor. What I got instead puzzles me and I'm hoping somebody here can explain what's happening in my circuit. I have included a link to a video where you can see the behavior in action.

Video:

Essentially, there was no response when hooking up 5V through the capacitor but I got a response by touching the top of the capacitor with my finger despite it being hooked up to nothing. I assume the capacitor was storing charge and I was just assisting in making a connection through the breadboard so I pulled the capacitor out and re-seated it and I'm sure there is a good connection but I got the same result.

Thanks in advance for helping out a novice. I'm sure the answer will be embarrassingly simple but I haven't been able to figure it out on my own and google isn't much help on this one.

EDIT: I should have been clearer in the video: The LED anode is connected to the emitter and the cathode to ground through a 330ohm resistor. The transistor is an S8050
 
Last edited:

Thread Starter

TarenF

Joined Jul 10, 2018
6
Please forgive the inexperience. In between crashes this is what I made. I'm sure there is a better way to indicate gnd and 5v but I haven't learned it yet.mystery Circuit schm.png mystery Circuit schm.png
 

BR-549

Joined Sep 22, 2013
4,938
I'm gonna guess that you're humming. Can you power the breadboard power supply with a nine volt battery or a battery pack? Can you turn all power strips off and wall plugs out.....without too much inconvenience?

And turn off all lights. Now repeat the experiment.....and see if it still lights.

Your body capacitance is charging you at a line frequency rate. It's enough to switch the transistor when touching the base.
 

Thread Starter

TarenF

Joined Jul 10, 2018
6
Thanks for your help.

I don't really have the capability to power the breadboard directly off a 9v but I did try it off of my arduino instead of the power supply in the video. The odd behavior stopped. The LED lit one last time and dimmed out quickly and now it doesn't light when I touch the capacitor.

This is fascinating. What exactly is "humming", am I really a source of enough voltage to switch a transistor? Why would that be the case off of a power supply but not the arduino?
 

BR-549

Joined Sep 22, 2013
4,938
Put your finger on the input of an audio amp. You will hear a "power supply hum".

Your body has a capacitance. You are a bag of salt water.

Google "body capacitance".
 

dl324

Joined Mar 30, 2015
13,506
Please forgive the inexperience.
I understand that you're new, inexperienced, etc, but do yourself a favor and study some schematics so you don't embarrass yourself with non-existent schematic drawing skills.

This is your schematic drawn more conventionally:
upload_2018-7-10_17-32-51.png
We strongly prefer for the "flow" to be top to bottom and left to right.

You mentioned a 100uF cap, but you labeled it as 0.1mF. Technically, that's correct, but mF is a seldom used suffix.
 

Thread Starter

TarenF

Joined Jul 10, 2018
6
I understand that you're new, inexperienced, etc, but do yourself a favor and study some schematics so you don't embarrass yourself with non-existent schematic drawing skills.

This is your schematic drawn more conventionally:
View attachment 155990
We strongly prefer for the "flow" to be top to bottom and left to right.

You mentioned a 100uF cap, but you labeled it as 0.1mF. Technically, that's correct, but mF is a seldom used suffix.

Thanks for redrawing that. That's definitely a form I would have preferred but I couldn't figure out the features of fritzing in between rapid crashes. I didn't want to label it 0.1mF but it wouldn't allow anything else. Typing 100 beside a uF would never go above 10. I tried the fritzing website but they are still using captcha 1.0 which isn't supported so you can't make a new account. Essentially I needed something other than fritzing. Any recommendations appreciated.

EDIT: I couldn't even get the labels upright, it took 2 crashes for me to give up on that idea.

@BR-549
Thank you very much for helping out, I'll do some research on that.
 

dl324

Joined Mar 30, 2015
13,506
Any recommendations appreciated.
If you want a schematic editor, many members seem to like KiCad. I prefer Eagle, but many say it's too hard to learn.

Paper and pencil will allow you to draw whatever you want.

If you want a circuit simulator, LTSPICE is free and popular. If you're learning, I think it would more helpful if you just bit the bullet and learned the basics.
 

ebeowulf17

Joined Aug 12, 2014
3,282
Thanks for redrawing that. That's definitely a form I would have preferred but I couldn't figure out the features of fritzing in between rapid crashes. I didn't want to label it 0.1mF but it wouldn't allow anything else. Typing 100 beside a uF would never go above 10. I tried the fritzing website but they are still using captcha 1.0 which isn't supported so you can't make a new account. Essentially I needed something other than fritzing. Any recommendations appreciated.

EDIT: I couldn't even get the labels upright, it took 2 crashes for me to give up on that idea.

@BR-549
Thank you very much for helping out, I'll do some research on that.
I'm a big fan of DipTrace for schematics and PCB layout (free version has plenty of capability for most hobby use.)

LTspice is also free and is an amazing tool. I can't imagine learning electronics without it (guess I'm lucky I didn't get into this 30 years earlier!) Its schematics aren't quite as pretty, but are perfectly fine. Probably 2/3 of the schematics you see in this forum are from LTspice. I highly recommend downloading it and starting to tinker. It can seem a little cumbersome at first, but don't get discouraged. It's worth the effort!
 

dl324

Joined Mar 30, 2015
13,506
I can't imagine learning electronics without it (guess I'm lucky I didn't get into this 30 years earlier!)
This is a double edged sword. I learned electronics without using a simulator just like I learned to do basic math without a calculator.

If you depend too heavily on simulators (or calculators), they'll do most of the thinking for you and whatever skills you had will atrophy.
 

ebeowulf17

Joined Aug 12, 2014
3,282
This is a double edged sword. I learned electronics without using a simulator just like I learned to do basic math without a calculator.

If you depend too heavily on simulators (or calculators), they'll do most of the thinking for you and whatever skills you had will atrophy.
Very true - I try to avoid using a simulator to generate new circuit concepts or figure out component values. I usually brainstorm in my head or on paper, and calculate values by hand. I use the simulator just to "build" and test ideas that I think I've figured out.

My workflow is essentially the same as it would be with physical parts, except that I don't have to order buckets of parts all the time, and changing connections when I discover a mistake takes only seconds.

But I'll admit that I've used spice as a crutch a few times - I can see where that would be an easy trap to fall into more regularly, and you're wise to caution against it!
 
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